The papermaking industry as well as other industries have long sought methods for enhancing the strength of products formed from fibrous materials such as, for example, paper and board products formed of cellulose fiber or pulp as a constituent. The dry-strength and related properties of a sheet formed from fibrous materials are especially important for various purposes. The problems and limitations presented by inadequate dry-strength have been particularly acute in the numerous industries where recycled furnish or fiber mechanically-derived from wood is utilized in whole or in part. In the papermaking industry for example, recycled cellulose fiber is typically used in the manufacture of newsprint and lightweight coated papers. These recycled fibers, however, are of a generally shorter length than chemically-pulped fibers. Paper produced from the shorter length recycled fibers have been found to have relatively poor dry-strength properties in comparison to paper manufactured from virgin, chemically-pulped fiber. The use of virgin chemically pulped fiber for all paper and board production, however, is extremely wasteful in terms of natural resource utilization and is cost-prohibitive in most instances and applications.
Various methods have been suggested in the past for improving the dry-strength and related properties of a sheet formed from fibrous materials such as paper or board materials formed of cellulose fiber. One method known in the art for improving the dry-strength properties of paper products, for example, involves the surface sizing of the sheet at a size press after its formation. While some of the critical properties of the product may be improved through sizing the surface of the sheets, not all equipment is amenable for such processes. Many papermaking machines, for example, including board and newsprint machines, are not equipped with a size press. Moreover, only the properties of the surface of the sheet are appreciably improved through surface sizing. Surface sizing, therefore, is either not available to a large segment of the industry or is inadequate for purposes of improving the strength of the product throughout the sheet. The latter factor is especially significant since paper failures during printing, for example, are obviously disruptive to production cycles and can be extremely costly.
A well-known method for increasing the strength of the paper product, without surface sizing of a sheet, is by lamination. Laminating is the process of applying a film to either one side or both sides of a pressed paper product. Lamination has been found to add stability to the sheet, allowing it to be more durable or stand upright. There are two major lamination categories: pouch and roll. Pouch lamination films are like envelopes and are sealed on one edge. Roll lamination films can involve a process in which a layer of film is applied to the front side of a document or it can involve a process in which the document is sandwiched between two layers and sealed by various lamination seal methods. The two most common methods of lamination are thermal lamination, which requires a heat source and pressure during the lamination process, and cold lamination, in which only one side of a document is laminated. The film used for cold lamination is much more costly than for thermal lamination, but the equipment is known to be less expensive. Additionally, cold lamination may not be as permanent as thermal lamination. Regardless of the lamination type or process utilized, lamination is known to be a costly method of adding strength to the paper product. It requires additional equipment, sealants, and films, and can introduce operational challenges to production time and quality control. Additionally, the lamination layer or layers contribute to the total finish caliper of the paper. Because total finish caliper of the paper is also an important consumer characteristic, processes which employ a lamination step are often restricted to using lower basis weight paper.
Another method to increase the strength of a paper product is through the addition of chemical additives directly to the fiber furnish prior to forming the sheet. One such process is taught by U.S. Pat. No. 5,328,567 to Kinsley, Jr. Common additives at the wet-end of a paper machine, for example, include cationic starch or melamine resins. The problem presented by these known wet-end additives used in the papermaking industry, however, is their inability to dramatically improve the mechanical properties of the paper in the Z-direction, such as peel strength, surface pick resistance and Scott internal bond. Another problem presented by such known wet-end additives is their relatively low degree of retention on the cellulose fiber during the initial formation of the sheet, at the wet-end of the paper machine. In most applications, significant portions of the wet-end additives accompany the white water fraction as it drains through the wire. This is due to high dilution and the extreme hydrodynamic forces created at the slice of a Fourdrinier machine. Alternatively, a significant portion of the additive may be lost in solution during the dwell time between its addition to the stock and the subsequent formation of the sheet on the machine. Accordingly, the use of known methods for internally strengthening fiber products have not produced a paper product with improved stiffness without the high costs and operational challenges associated with a lamination process.
Crosslinkers have been used in the paper-making industry. For example, U.S. Pat. No. 5,281,307 to Smigo et al. uses a crosslinking agent along with a polyvinyl alcohol/vinylamine copolymer containing between 0.5 and 25 mole % vinylamine units to improve certain properties of paper. In addition, GB Patent No. 1,471,226 relates to a process for the preparation of an aqueous dispersion of modified cellulose fibers, which comprises the steps of: (a) treating cellulose fibers, in aqueous dispersion, with a crosslinking agent capable, on the application of heat, of crosslinking cellulose fibers, (b) heating the dispersion to effect at least partial crosslinking of the cellulose fibers, and (c) treating the dispersion of at least partially crosslinked cellulose fibers with a polymer containing hydroxyl and/or amino groups. The desired paper product produced according to the '226 patent is to minimize jamming in a copying machine and therefore has a basis weight of preferably from 25 to 90 g/m^2 (i.e., 0.00512 lbs/ft^2 to 0.0184 lbs/ft^2).
U.S. Pat. No. 6,379,499 to Yang et al. discloses a method of treating paper comprising: contacting the paper with a hydroxy-containing polymer and a multifunctional aldehyde, in the presence of a catalyst in some embodiments. The multifunctional aldehyde may be gluteraldehyde, and the hydroxy-containing polymer may be polyvinyl alcohol. Yang teaches a process in which the multifunctional aldehyde and polyvinyl alcohol are pre-mixed (i.e., mixed together prior to their addition to the paper-making process). The multifunctional aldehyde of Yang is used to at least partially crosslink the polyvinyl alcohol, not the starch or pulp fibers, before the multifunctional aldehyde and the polyvinyl alcohol are added to the wet end pulp slurry. As Table 3 of Yang shows, the pre-mixing and crosslinking of gluteraldehyde and polyvinyl alcohol is necessary to retain or improve the dry strength and folding endurance of the resulting paper in the process according to Yang. With increased gluteraldehyde, however, the folding endurance is significantly decreased as a detriment to the desires of Yang. High amounts of multifunctional aldehydes have generally be found to exhibit a loss of dry strength and decreased folding endurance, which is in accordance with the findings of Yang, but has now been employed to produce a rigid sheet while retaining or improving stiffness.
While research into improving the mechanical properties of the paper in the Z-direction, surface pick resistance, and Scott internal bond remains on-going, there has recently been the emergence of alkaline papermaking processes to solve other unmet operational needs. Recent technologies employ a neutral or alkaline papermaking process, which is carried out at pH 6 to 10, instead of an acidic papermaking process. The neutral or alkaline papermaking process has many advantages over known acidic processes, such as, for example: (1) smaller energy utilization; (2) reduced corrosion of machinery; and (3) environmental benefits associated with the non-acidic white water system and waste stream.
In the conversion from acid papermaking to alkaline papermaking, customers often complained that the resulting paper product lost stiffness. Tests have shown that this loss was in the rigidness of the paper sheet, not in the actual stiffness measurements of the products. This is often described as a loss of snap or rattle in the paper product. As is known in the art, “rigidness” relates to the brittleness of a paper product (i.e., flexural stiffness or flexural rigidity), while “stiffness” relates to the bending resistance of the paper product. A loss in rigidness is an increase in the paper product's flexibility, but a loss in stiffness is a decrease in the amount that the paper product resists bending. To achieve a low thickness (e.g., low caliper) paper product with the necessary stiffness and rigidity, paper producers have had to thus far laminate sheets of lesser caliper together. However, this adds a substantial and costly step to the paper-making process and can not be utilized for all paper products as lamination increases the overall basis weight of the paper product.